The present invention relates to a method of forming insulating films in general. More particularly, it relates to such a method of sputtering suitable for forming excellent dielectric films suitable for use in capacitances.
In the recent years, dielectric (insulating) films deposited by CVD have been utilized to form capacitances for use in integrated semiconductor devices. The employment of CVD makes it possible to deposit dielectric films at low temperatures up to 450.degree. C. so that inexpensive substrates such as soda lime glass or borosilicate glass substrates can be used. Similar low temperature deposition can be accomplished also by plasma CVD and sputtering in an atmosphere comprising an inert gas such as argon at a density of 100% to 80%. The use of argon has been known to increase the sputtering yield.
In accordance with experiments of the inventor, it has been found that the number of the interface states occurring between the dielectric film and the underlying electrical active region seriously depends upon the argon density of the sputtering atmosphere. A conspicuous example is the case of dielectric films made of tantalum oxide. In this case, many clusters of tantalum atoms of 5 to 50 .ANG. diameter are formed in the oxide film due to stability of metal tantalum. It has been also found that the argon density significantly influences the difference in flat band voltage from the ideal value which indicates the degradation of the film and reflects the state number of fixed charge and the clusters.
There are other attempts to form dielectric films by the use of photo-CVD. In this case, the underlying surface is less damaged and the density of interface states is as low as 2.times.10.sup.10 eV.sup.-1 cm.sup.-2. On the other hand, the deposition of photo-CVD takes much time to complete due to very slow deposition speed so as not to be utilized for massproduction. Furthermore, the long-term reliability is not sure because of hot-electron effect resulting from hydrogen utilized during deposition.